Black Tank Slow Leak

[W5LEW]

Yesterday, we returned home to smell apparently a black tank odor in the kitchen when we walked in. I found the smell to be coming from the pocket door, not the toilet. Storage bay investigation with flashlight found some black water drip on the plywood base for the water pump, the apparent cause of the smell. Right now, I figure the leak is coming from the 3" PVC pipe/tank connection above and to the curbside of the water pump. Am going back this morning to take some pictures with my phone if I can get in there to confirm my suspicions.

If I'm right, what kind of sealant do I need to use to seal the connection and stop the leak? Not sure if I can get my caulk gun in there. Other other sealing options?

BTW, the tank is full - time for a dump due today. Also, my BIL and I replaced the black and gray valves a few months ago. Wonder if we could have put the black tank/pipe connection in a bind or something putting in the new valve that could have caused this?

Thanks for your help.